Hi Bernd,

> Indeed there is a setting for drawing the constellation boundaries.
> Right click (as mostly in Guide to change the appearance) on one
> boundary and change the "Show at" to lets say 0 - 180...

As you found out, there is indeed a problem in that the lines
still disappear after a certain point. And there is a problem beyond
this: they are stored with an precision in RA of 1/40 degree, and
in dec of one arcminute.

This is code I wrote back in the early 1990s (!), and I no longer
recall what my reasoning was when I did this. I was probably trying
too hard to conserve bytes and to avoid floating-point arithmetic.
(The first few versions of Guide were written on a 486 with no math
chip, and probably run on even lower-end machines. So Guide did a lot
of fancy things to ensure that math could be done on an integer basis
as much as possible... thank Intel and AMD for math coprocessors; such
concerns are almost as obsolete as a slide rule these days.)

However, none of this affects the problem Luc is mentioning, because
we've been checking to see the instant at when the B1875 RA of Venus
crosses the value 7h 48.5m. Guide is quite confident that this happens
eighteen minutes later than JPL is getting.

Using Horizons to generate a minute-by-minute ephemeris with the
constellation ID included yields the following output... which matches
Luc's calculation for the moment Venus enters CMi. (And again, these
J2000 RA/decs match those computed by Guide. I can't get Horizons
to produce B1875 data.)

It also looks as if the JPL folks have been thinking about these
matters recently; http://ssd.jpl.nasa.gov/?horizons_news has an
update on 15 February mentioning that "...constellation boundaries
have been updated to use two more digits of precision for sub
0.1-arcsecond specification." (Which also means that if they were
using apparent B1875.0 coordinates instead of mean B1875.0 ones,
the error could have been overlooked, masked by the greater error
due to using imprecise boundaries.)

-- Bill

Date__(TT)__HR:MN R.A._(ICRF/J2000.0)_DEC Cnst
***************************************************
$$SOE
2557-Aug-14 12:20 07 55 30.79 +13 07 35.5 Cnc
2557-Aug-14 12:21 07 55 30.72 +13 07 35.7 Cnc
2557-Aug-14 12:22 07 55 30.65 +13 07 35.8 Cnc
2557-Aug-14 12:23 07 55 30.58 +13 07 36.0 Cnc
2557-Aug-14 12:24 07 55 30.51 +13 07 36.2 Cnc
2557-Aug-14 12:25 07 55 30.44 +13 07 36.4 Cnc
2557-Aug-14 12:26 07 55 30.37 +13 07 36.5 Cnc
2557-Aug-14 12:27 07 55 30.30 +13 07 36.7 Cnc
2557-Aug-14 12:28 07 55 30.23 +13 07 36.9 CMi
2557-Aug-14 12:29 07 55 30.16 +13 07 37.1 CMi
2557-Aug-14 12:30 07 55 30.09 +13 07 37.2 CMi
2557-Aug-14 12:31 07 55 30.02 +13 07 37.4 CMi
2557-Aug-14 12:32 07 55 29.95 +13 07 37.6 CMi
2557-Aug-14 12:33 07 55 29.87 +13 07 37.7 CMi